The present invention relates to a method of carrying out an amplification reaction and in particular a polymerase chain reaction (PCR) using an internal temperature control mechanism.
A common problem in biochemical reactions, in particular miniaturised biochemical reactions is controlling the temperature. Invasive temperature probes add to the thermal mass of the sample and increase time constraints associated with heating and cooling. A particular example where such a problem occurs is with minaturised amplification reactions such as the PCR reaction. In this reaction, cycling between various accurate temperatures is an essential element. In outline, the procedure consists of the following steps, repeated cyclically. Denaturation : A mixture containing the PCR reagents (including the DNA to be copied, the individual nucleotide bases (A,T,G,C), suitable primers and polymerase enzyme) are heated to a predetermined temperature to separate the two strands of the target DNA.
Annealing: The mixture is then cooled to another predetermined temperature and the primers locate their complementary sequences on the DNA strands and bind to them.
Extension: The mixture is heated again to a further predetermined temperature. The polymerase enzyme (acting as a catalyst) joins the individual nucleotide bases to the end of the primer to form a new strand of DNA which is complementary to the sequence of the target DNA, the two strands being bound together.
Any interference with the reaching the predetermined temperatures as a result of the temperature measurement can present a significant problem in terms of the success of the amplification reaction.
The applicants have found a way in which the temperature present in a biochemical reaction can be monitored without the need for the application of temperature probes.
According to the present invention there is provided a method of monitoring the temperature of a biochemical reaction, said method comprising effecting the reaction in the presence of a fluorescently labelled temperature probe DNA sequence which comprises a double stranded region which denatures at a predetermined temperature, the fluorescent label or said temperature probe sequence being arranged so that the nature of the fluorescence changes at the point at which denaturation of the said region takes place; and monitoring fluorescence from said reaction mixture so as to determine when the said predetermined temperature has been reached.
The labelled temperature probe DNA sequence added to the reaction mixture in the method acts as a temperature probe allowing the temperature of the reaction to be accurately set without requiring external temperature probes.
The temperature probe DNA sequence may comprise a double stranded DNA sequence, or it may be in the form of a single nucleic acid strand, end regions of which hybridise together so as to form a loop or xe2x80x9chairpinxe2x80x9d structure.
Suitable fluorescent labels include intercalating dyes, which are interposed between the strands of a double stranded region of a DNA sequence. When the double stranded DNA region containing the intercalating dye reaches the predetermined temperature, it will be denatured, thus releasing the intercalating dye present between the strands. At this point the fluorescence from the mixture will reduce significantly, giving a readable signal.
The process using a double stranded DNA sequence as a temperature probe is illustrated diagrammatically in FIG. 1 hereinafter.
When intercalating dye (2) is added to a solution of double stranded DNA (1), it becomes interposed between the strands. The concentration of the dye (2) in this way produces a recognisable signal. On heating of the DNA so that it is denatured, dye is released and this event can be witnessed. Cooling to a temperature at which the said sequence will anneal again results in the intercalating dye becoming again trapped between the strands (see FIG. 1).
Suitable intercalating dyes include SYBRGreen(trademark), SYBRGold(trademark) and ethidium bromide or other commercially available dyes.
Alternatively, the fluorescent label used in the method of the invention may utilise fluorescence resonance transfer (FRET) as the basis of the signal. These labels utilise the transfer of energy between a reporter and a quencher molecule. The reporter molecule is excited with a specific wavelength of light for which it will normally exhibit a fluorescence emission wavelength. The quencher molecule is also excited at this wavelength such that it can accept the emission energy of the reporter molecule by resonance transfer when they are in close proximity (e.g. on the same, or a neighboring molecule). The basis of FRET detection is to monitor the changes at reporter and quencher emission wavelengths.
For use in the context of the present invention, the DNA sequence used as a temperature probe can be provided with a reporter and a quencher molecule, arranged so that the hybridisation of the strands alters the spatial relationship between the quencher and reporter molecules. Examples of such arrangements are illustrated in FIG. 2 and FIG. 3.
FIG. 2 illustrates an Example where the temperature probe sequence is a single stranded xe2x80x9chairpinxe2x80x9d type sequence (3), where the end portions hybridise together. A reporter molecule (4) is attached in the region of either the 5xe2x80x2 or the 3xe2x80x2 end of the sequence and a quencher molecule (5) is attached at the opposite end such that they are brought into close proximity when the sequence is in the form of the loop. In this arrangement, FRET occurs and so fluorescent signal from the reporter molecule is reduced whilst the signal from the quencher (5) molecule is enhanced.
On denaturation however, the opposed end regions of the sequence separate so that the reporter and quencher molecules become spaced and so FRET no longer occurs. This changes the signals from the respective molecules and so this event can be detected.
Another arrangement is illustrated in FIG. 3. In this case, the reporter (4) and quencher molecules (5) are located on different strands (6,7 respectively) of a DNA temperature probe sequence and are located such that on hybridisation of the strands, they are brought into close proximity to each other so that FRET can occur.
Yet a further embodiment is illustrated in FIG. 4. In this case, an intercalating dye (2) is used as an element of the FRET system. A quencher molecule (5) which can absorb radiation from the dye may be arranged on a strand of the temperature probe sequence such that it can absorb radiation from dye which is close proximity to on hybridisation of the strands. When the temperature probe sequence reaches a temperature at which it is denatured, the dye (2) is dispersed and so the signal from the quencher molecule (5) changes.
This embodiment is advantageous in that only a single label need be applied to the temperature probe sequence. Single labelled sequences of this type are more economical to produce.
In yet a further embodiment (FIG. 5), the reporter (4) and quencher (5) molecules are positioned on two oligonucleotide strands (9 and 10 respectively) which do not hybridise together. They are however designed so that in use, they hydridise to a DNA sequence present in the reaction mixture, which may be a plasmid (11), such that the reporter (4) and quencher (5) are brought into close proximity and FRET can occur between them, giving a recognisable signal.
The DNA sequence to which they bind may be part of the reaction system, for example where the reaction being monitored is a PCR reaction wherein the DNA sequence comprises or is part of the amplification target sequence. Alternatively, the sequences may be added to the reaction in order to provide the basis for the temperature probe of the invention.
The temperature probe sequence of the invention may be designed so that it denatures at any desired predetermined temperature. For example, the denaturation temperature of a sequence depends to some extent on its length. Longer sequences will denature or melt at higher temperatures. Furthermore, it is known that the bases C and G bind together more strongly than A and T. Therefore, the greater the higher the content of the bases G and C contained within a sequence, the higher the melting point of the sequence will be. This feature is illustrated in FIG. 5 which shows the melting temperature of a DNA sequence plotted against the percentage of and GC base pairs which are present within in. Thus, by adjusting the GC content, the temperature probe sequence may be designed so that, if desired, it also has a predetermined length.
The method of the invention is particularly applicable for use in amplification reactions such as the polymerase chain reaction (PCR). In this case, the temperature probe sequence of the invention is introduced into the reaction vessel. Suitably the temperature probe sequence is designed such that it generates a detectable signal when it reaches the optimum annealing temperature of the target DNA sequence as this is intermediate temperature is most difficult to set accurately in practice. However, more than one such temperature probe sequence may be added and arranged to provide appropriate and preferably different signals when the predetermined extension and/or denaturation temperatures have been reached.